1. Field of Invention
This invention belongs to the area of flat panel display and more specifically to the backlight technology for Liquid Crystal Display (LCD).
LCD does not emit light and hence requires a backlight for its function as a visual display. Backlights based on Cold Cathode Fluorescent lamps (CCFLs) have been employed for backlighting LCDs. Recently Light Emitting Diodes (LEDs) have been employed as light sources for backlighting LCDs. White LEDs are predominantly used in cell phones, digital cameras, iPads, lap top computers and LCD TV for backlighting LCDs. Red, Blue and Green LEDs are used also used in backlighting LCD TV. LCD requires a flat or sheet source of light at its rear side. As LEDs are point sources of light, there is a need to convert the point source of light into a sheet source of light. This conversion is done through two configurations namely, edge-lit configuration and direct-lit configuration. In an edge-lit configuration, the LEDs are assembled at the edges of a rectangular light guide, usually a polycarbonate slab, and the light from LEDs undergo total internal reflections at the inner surface of light guide and finally get extracted through top surface of light guide facing the back surface of LCD. In the direct-lit configuration, LEDs are assembled inside a box and a diffuser sheet is assembled over the LEDs. Light from LEDs get mixed inside the box and further mixed in the diffuser sheet and finally emanates from the diffuser sheet towards LCD.
Thus a sheet source of light, usually white light, is generated and provided as backlight for LCD. The white light from LEDs contains all three primary colors and the colors are further filtered by color filters incorporated inside LCD for generating good quality of color pictures. It is well known that the color filters absorb 70-80% of incoming light and hence the LCD becomes optically less efficient. This still remains a challenge for the LCD industry. There is one more challenge that the LCD industry is facing and that is related to the polarizer employed for the functioning of LCD. A polarized light is necessary for the functioning of LCDs employed in major applications namely, TV, Cell phone, Desk top computer, lap-top computer, tablet computer and monitors. For polarizing the light a polarizer is laminated to the rear substrate of LCD facing the backlight and the light from the backlight is thus polarized by the polarizer before the light enters the liquid crystal layers. There is an analyzer (also possess polarizing property) laminated to the front substrate of LCD and this analyzer does not impose light loss. In all the descriptions that follow, the polarizer is the main focus and not the analyzer. The polarizer absorbs 50% of the light incident on it. Finally only 5-6% of the light sent by backlight to LCD emerges for the viewer to see the pictures or any information on LCD screen and in this manner LCD is lacking optical efficiency.
2. Description of Prior Art
No single prior art, to the best of knowledge of the current inventors, deals with the solution for optical inefficiency of LCD caused by both the polarizer and color filters, simultaneously. There are prior arts that deal with the elimination of color filters alone inside LCD. For example in one prior art (U.S. Pat. No. 4,978,888), Anandan et. al describes the use of flat fluorescent lamp incorporating red, blue and green color phosphors in the form of pixels, for backlighting color filter-less LCD. Red, blue and green color light from the pixilated surface of the flat fluorescent lamp provides individual primary colors of light to the pixels of LCD that does not incorporate color filters. Hence the use of the term ‘color pixel backlighting’. The drawback in this prior art is the high ignition voltage in the range of thousands of volts and running voltage in the range of hundreds of volts, in addition to thick and massive glass plates employed for the flat light source.
In another prior art (M. J. J. Jack et. al—“Color Separating backlight for improved LCD efficiency”, pp. 175-178, Proceedings of 27th International Display Research Conference, 18-20 Sep. 2007, Moscow, Russia; Yoichi Taira et. al, “Low power LCD using a novel optical system”, SID-02 Digest, pp. 1313-1315, 2002; Jyrki Kimmel et. al, “Diffractive backlight grating array for mobile displays”, Proceedings of IDRC 2007, pp. 171-174, Moscow, September 2007) the authors describe the use of diffraction grating over the light guide to separate the three primary colors from the white light and employ a sheet of micro-lenses. The diffraction grating-separated three primary colors go through the LCD that has no color filters. The drawback in this prior art is the ‘color cross-talk’. That is, light of one color passes through the unintended pixel of LCD where a different colored light is incident. It is clear from the prior art that providing a simple solution for ‘color pixel backlighting’ for LCD, not requiring color filters, is a challenge.
In yet another prior art (U.S. Pat. No. 7,036,946 B1) Donald E. Mosier describes LCD backlight employing ultraviolet LEDs. In this invention, ultraviolet light from LEDs is coupled to a light guide and the ultraviolet light is made to fall on phosphor coated on the surface of light guide to emit visible light towards the LCD. This invention is completely devoid of the concept of ‘color pixel backlighting’ and does not deal with LCD not requiring color filters.
In a research paper (N. Ogawa et. al—“Filed sequential color, LCD using organic EL backlighting”—SID Digest of Technical Papers SID International Symposium 1999), N. Ogawa et. al describe organic based electroluminescent backlight that is backlighting an LCD without color filters. This art is based on the field sequential operation of LCD wherein each frame of LCD driving consists of only one color. For example the backlight will provide a sheet of red light to the LCD 60 times per second and the LCD will select red pixels during this period. The next will be green sheet of light from backlight for 60 times a second followed by blue sheet of light. In other words the backlight will provide continuous sheet of light with a frame rate of 60 Hz for each color. This takes place sequentially for each color during which time the desired color pixels will be selected on LCD to generate color images. In this art, there is no ‘color pixel backlighting’ that takes place continuously. This is totally a different mode of operation and the sequential flashing of color sheet of light on LCD results in Psycho-physical effect that makes display exhibit ‘color break-up’, a defect that is still to be solved by LCD industry to make any commercial product.
In the literature, employment of quantum rods for LCD backlight has been described. One such literature (Zhenyue et. al—“Emerging Quantum-Dots-Enhanced LCDs”, IEEE Journal of Display Technology, Vol. 10, No. 7, July 2014) describes various configuration of backlight employing quantum rods but all the described configurations need color filters inside LCD.
Now turning to the polarizer of LCD, there have been several attempts to eliminate the polarizer laminated to LCD and thus increase the optical efficiency of LCD. One prior art reported in the literature (Shin-Tson Wu-“Energy efficient TFT-LCDs”, p. 20 on wire grid polarizer, Seminar Lecture Notes, Society for Information Display, Jun. 1, 2009) is the use of wire-grid polarizer in place of polarizer laminated to LCD. This is still another additional component namely removing the polarizer sheet and adding wire-grid polarizer. There is also additional disadvantage and that is 30% of the light is wasted by way of unwanted polarization. Another method proposed (by S. M. P. Blom et. al—“Towards polarized light emitting backlights: “Micro-structured anisotropic layers” Asia Display/IDW'01 Proceedings, pp. 525-528, 2001, Nagoya, Japan) to eliminate polarizer of LCD is to incorporate an anisotropic layer of material as an integral part of backlight. In this method, the purity of polarization is absent and further three is light loss through scattering effects. Hence this method did not succeed.
In all the foregoing inventions it is clear that none of the methods could solve the problem of optical inefficiency present in LCD. There is one invention (U.S. Pat. No. 8,459,855B2) titled, “ultraviolet LED based color pixel backlight incorporating quantum dots for increasing color gamut of LCD” by Anandan et. al that has the correct design and configuration to eliminate color filters for LCD and hence the optical efficiency of LCD can be improved. But this art still requires polarizer for LCD. That is, this invention can solve one problem (color filter) but not both (color filter and polarizer). It is also clear from the foregoing inventions that there is not a single invention that can eliminate both the polarizer and color filters of LCD to enhance the optical efficiency of LCD.